1. Field of the Invention
The present invention relates to an optical scanning device and an image forming apparatus.
2. Description of the Related Art
In image forming apparatuses such as digital copiers and laser printers, high-density image formation using optical scanning has been progressing, and a small-sized beam spot on a photosensitive element is demanded of the image forming apparatus to meet the progress. In addition, there is a continuous demand for high-speed image formation, and thus it is desired to realize an optical scanning device capable of meeting these demands. As a means to achieve high-speed and high-density image formation, rotation speed of a polygon scanner may be increased. With such means, however, there arise problems of increase in power consumption and noise, heat generation, and degradation of durability.
As another means to achieve high-speed and high-density image formation, an image forming apparatus using a multibeam writing system has been put to practical use. More specifically, the multibeam writing system uses a light source including a plurality of light emitting points, and scans one surface with a plurality of beams. The light source including the light emitting points on one element, however, has some problems as follows:    1. Heat generated by electrically driving light-emitting elements causes crosstalk between the light-emitting elements, and the crosstalk makes their property unstable.    2. The more the number of light-emitting elements increases, the farther light emitting points are apart from the optical axis of an optical system. Therefore, it is difficult to ensure optical characteristics for the entire light fluxes. The optical characteristics mentioned here indicate light-use efficiency, field curvature, and the like.
The following are known as technologies related to solution of these problems.
Japanese Patent Application Laid-Open No. 2001-272615 discloses a technology for reducing influence of thermal crosstalk by providing light emitting points at equal spacing and improving an accumulation rate without bringing spaces between the light emitting points closer to each other.
Japanese Patent Application Laid-Open No. 2001-350111 discloses a technology for obtaining high-density images by reducing non-uniformity of density of a recorded image while ensuring scanning speed to avoid the problem of thermal crosstalk. More specifically, the non-uniformity is reduced by determining each space between light emitting points of a light source and a direction in which the light emitting points are arrayed in a multibeam scanning device.
Japanese Patent Application Laid-Open Nos. 2004-126192 and 2005-258392 disclose a technology for reducing variation of a focus position due to temperature change by combining a diffractive surface and a refractive surface in an optical system provided in an upstream side of a deflector.
Japanese Patent Application Laid-Open Nos. 2003-337295 and H11-223783 disclose a technology for correcting variation of a focus position, due to a scanning optical system in association with temperature change, by changing the power of a diffractive portion.
Japanese Patent Application Laid-Open No. 2002-214556 discloses a technology for correcting variation of a focus position due to temperature change by combining at least three lenses in an optical system provided in an upstream side of a deflector without using a diffractive surface.
Japanese Patent Application Laid-Open No. H10-333070 discloses a technology for correcting variation of a focus position by providing a diffractive optical surface on a scanning lens.
The technologies described in Japanese Patent Application Laid-Open Nos. 2001-272615 and 2001-350111 deals with the first problem, but both the patent documents do not teach or suggest the solution to the second problem. Japanese Patent Application Laid-Open No. 2004-126192 also does not teach or suggest the solution to the problem addressed by the present invention.
Japanese Patent Application Laid-Open No. 2005-258392 mentions the multibeams, but does not teach or suggest the solution to the problems when the multibeams are used.
Japanese Patent Application Laid-Open Nos. 2003-337295 and H11-223783 also do not mention anything about the multibeams such that the one photosensitive element is scanned with the beams.
It is desirable to push ahead resinification of a constructed lens to achieve low cost of an optical scanning device and an image forming apparatus using the same. As for a glass lens, the curvature of a lens surface and the thickness and refractive index of the lens vary caused by the change of environmental temperature, and the refractive index of the lens varies caused by variation of oscillation wavelength of a semiconductor laser which is the light source. As a result, a focus position changes, and a spot diameter thereby increases, resulting in image degradation. As for a resin lens, the curvature of a lens surface and the thickness and refractive index of the lens vary more largely than these of the glass lens caused by the change of environmental temperature, and the refractive index of the lens varies more largely than that of the glass lens caused by the variation of wavelength of a semiconductor laser which is a light source. As a result, image degradation is largely affected thereby.
To solve this problem, there is the method of correcting variation of the refractive index by combining at least three lenses in the optical system provided in the upstream of the deflector as is described in Japanese Patent Application Laid-Open No. 2002-214556. However, this structure has a disadvantage such that the increase of the number of lenses causes cost increase. Moreover, the technology described in Japanese Patent Application Laid-Open No. 2002-214556 also requires one piece of glass lens, and this is also a factor of cost increase.
There is another method of correcting variation of the refractive index by providing a diffractive optical surface on the scanning lens as is described in Japanese Patent Application Laid-Open No. H10-333070. However, the scanning lens has a wide area for passage of light flux, and the diffractive optical surface has to be formed on the wide area, which takes time, and this causes cost increase.
In all the patent documents, there is no technological disclosure about the second problem which is specific to the multibeam. The diffractive optical surface has a specific shape such that each interval between diffraction gratings is shorter as the diffraction gratings are apart from the optical axis. Therefore, higher machining technology is required for an area which is farther from the axis, and this may easily cause manufacturing error to occur. If the manufacturing error occurs, then diffraction efficiency varies, which causes the light amount on the surface to be scanned to change for each light flux. Thus, images provided by the image forming apparatus have non-uniform density.